A device that utilizes energy of solar ray is roughly divided into a device that directly converts solar ray to electrical energy and a device that converts solar ray to thermal energy. A solar cell is an example of the former. Examples of the latter include the followings:
(A) a solar heat utilization system that produces warm water and warm wind utilizing solar heat and utilizes this for hot-water supply and air conditioning; and
(B) a thermoelectric generation system that generates temperature difference at both ends of a thermoelectric element utilizing solar heat and converts the temperature difference to electrical energy.
Of these, the solar cell can acquire only electric power and the solar cell cannot effectively utilize solar energy. A solar water heater has relatively high utilization efficiency of solar energy. However, since warm water is produced more than a required amount, the solar water heater cannot effectively utilize solar energy as a whole. In addition, the solar water heater can acquire no electric power. Further, solar thermal power generation using a steam turbine requires large-scale facilities.
In contrast, the thermoelectric generation system using the thermoelectric element has an advantage that the system can acquire both electric power and warm water and requires no large-scale facilities. Therefore, as for the thermoelectric generation system using the thermoelectric element, various proposals have been made.
For example, Non Patent Literature 1 discloses solar thermoelectric generators (STEGs) that convert solar ray to heat using a solar absorber put in a vacuum container and convert the heat to electric power by a thermoelectric element. The STEGs in the literature achieve thermoelectric conversion efficiency of 4.6%.
In the literature, thermoelectric power generation is performed using Bi2Te3 based thermoelectric materials. However, since the high-temperature side is approximately 200° C., no great temperature difference is produced in the thermoelectric element and the great enhancement of the thermoelectric conversion efficiency cannot be expected. Further, since heat unconverted to electricity is not utilized, the utilization efficiency of solar energy is low.
Non Patent Literature 2 discloses a calculation result of thermoelectric conversion efficiency of STEGs. The literature estimates that radiation of a high-temperature part can be inhibited by using a solar absorber composed of a long pass filter optimum for a wavelength of light and conversion efficiency of 15.9% at 1000° C. is acquired.
However, a long pass filter which has heat resistance and a cut-off wavelength of which is precisely controlled does not exist and the abovementioned estimate is not validated. Further, to acquire high thermoelectric conversion efficiency, temperature of the high-temperature part is required to be raised; however, a problem occurs that when the temperature of the high-temperature part is raised, heat loss by radiation increases.
Patent Literature 1 discloses a generator that heats the high-temperature side of a thermoelectric element using solar ray and cools the low-temperature side of the thermoelectric element using thermoelectric materials having Thomson effect.
The generator in the patent literature removes heat which is transmitted through the thermoelectric element without being converted into electric power by a system utilizing Thomson effect, and therefore utilization efficiency of solar energy is low.
Patent Literature 2 discloses power supply equipment where a thermoelectric element is arranged on an outer peripheral surface of a water pipe and solar ray irradiates the high-temperature side of the thermoelectric element using a crooked mirror surface body.
In the power supply equipment disclosed in the patent literature, when a degree of concentration of solar ray is low, the temperature of the high-temperature side of the thermoelectric element does not rise and thermoelectric conversion efficiency is low. In contrast, when the degree of concentration is high, heat loss due to convection in the high-temperature part cannot be prevented because the thermoelectric element is not put in a vacuum container. Further, the power supply equipment also does not have the function of preventing heat loss due to radiation.
Patent Literature 3 discloses a solar water heater where a solar heat collection face is provided to a circulating passage in which water in a warm water tank is circulated, warm water is produced from cool water using solar heat, the temperature difference between the cool water and the warm water is converted to electric power by using a thermoelectric element, and the warm water is forcedly circulated using the acquired electric power.
In the solar water heater disclosed in the patent literature, since all heat does not pass through the thermoelectric element, electric power is generated by only a small amount.
Patent Literature 4 discloses a hot-water supply system where solar ray is concentrated, electric power is generated by making infrared ray in the solar ray incident on a thermoelectric element by a wavelength selection mirror and making the remaining light incident on a solar cell and further, exhaust heat from the thermoelectric element and the solar cell is utilized for hot-water supply.
In the patent literature, no experimental results of the hot-water supply system are described. Further, in the hot-water supply system in the patent literature, temperature of the thermoelectric element does not rise and power generation efficiency of the thermoelectric element is low. Moreover, since temperature of exhaust heat from the solar cell and the thermoelectric element is low, efficiency of heat collection is unsatisfactory though a heat pump using carbon dioxide for coolant is used. Further, since the thermoelectric element and the solar cell are combined, structure of the system is intricate.
Patent Literature 5 discloses a thermoelectric cogeneration system that houses a heat storage material which can store solar heat as chemical energy in a reactor and converts heat generated in the reactor to electric power by the thermoelectric element.
As the thermoelectric cogeneration system disclosed in the patent literature performs heat storage and heat release using reversible reaction (for example, Mg(OH)2MgO+H2O) of the heat storage material, the system has a merit that a high-temperature part of the thermoelectric element is kept constant and electric power can be stably supplied. However, energy loss is caused in a heat storage process and utilization efficiency of solar heat energy is low.
Further, Patent Literature 6 discloses a method of converting solar ray to heat using a light absorber, converting the heat to electric power by a thermoelectric element, further making fluid (for example, water) flow on the low-temperature side of the thermoelectric element and heating the fluid.
As disclosed in the abovementioned related art literatures, when the light absorber and the thermoelectric element are combined, utilization efficiency of solar energy can be enhanced to a certain extent. Further, when heat wasted from the low-temperature side of the thermoelectric element is recovered by various methods, the utilization efficiency of solar energy is further enhanced.
In contrast, to further enhance the utilization efficiency of solar energy, it is preferable that temperature of a high-temperature part of the thermoelectric element is raised to be higher. However, since heat loss by radiation increases as the temperature of the high-temperature part is higher according to the previous method, the enhancement of the utilization efficiency has had a limit.